An ansa-metallocene is a catalytic compound containing two ligands that are linked by a bridging group. The link prevents rotation of the ligands and modifies the structure and reactivity of the metal center.
One potential application of ansa-metallocenes is in polymerization reactions for olefin homopolymers or copolymers. Particularly, ansa-metallocenes based on cyclopentadienyl-fluorenyl ligands have become of great significance in the polymerization of high-molecular weight polyethylenes with controlled microstructures. In addition, ansa-metallocenes containing indenyl ligands have excellent activity and can be used to produce polyolefins with high stereoregularity.
A metallocene, which is, per se, a complex soluble in a solvent, is a homogeneous catalyst. Meanwhile most Ziegler-Natta catalysts used in commercial manufacturing processes are of a heterogeneous type, with the active site metal on a solid support. Manufacturing processes are designed according to characteristics of Ziegler-Natta catalysts employed. For use in existing commercial manufacturing processes, metallocenes need transformation to a supported catalyst form in which a catalyst is affixed to a suitable support. Many studies have been reported on various supports and preparation methods of supported catalysts. Of them, many are directed to catalysts on supports such as silica, silica-alumina, and silica-magnesia.
Supported catalysts are a patent item of keen interest for commercialization. However, the application of metallocene catalysts to supports requires many troublesome processes, with the high likelihood of losing catalytic active sites depending on the states of the supports. In addition to this drawback, a limited amount of catalysts applied to supports results in low catalytic activity of the supported catalyst.
On the other hand, when a heterogeneous catalyst without a support is prepared by emulsion formation, a solvent, which is the third factor besides the catalytic factor and the cocatalyst, must be used, together with a surfactant, which may have adverse effects on the activity of the catalyst. A further disadvantage is the complicated process that a final catalyst product can only be separated by solidification following a separate emulsion formulation method.